Recently much interest has been directed towards diesel engines due to their inherent fuel efficiency and durability. However, diesel emissions have come under attack both in the United States and Europe, as being generally undesirable. As such, stricter environmental regulations will require diesel engines to meet higher emissions standards. Therefore, diesel engine manufacturers and emission-control companies are working to achieve diesel engines that are cleaner and meet the most stringent emission requirements under all operating conditions with minimal cost to the consumer.
One of the biggest challenges in lowering diesel emissions is controlling the levels of particulates present in the diesel exhaust stream. Diesel particulates are mainly composed of carbon soot. One way of removing such soot from diesel exhausts is through use of diesel filters. The most widely-used diesel filter is a diesel particulate filter (sometimes referred to as a “wall flow filter”) which filters the diesel exhaust by capturing the soot on or in its porous walls. The diesel particulate filter is designed to provide excellent soot filtration without significantly hindering the exhaust flow, i.e., without creating significant unwanted back pressure.
Within the industry, Silicon Carbide (SiC) has been used for certain diesel particulate filters. However, SiC is expensive, heavy and has a relatively high coefficient of thermal expansion (CTE). Thus, the use of SiC materials in diesel particulate filters requires expensive multi-component designs (sometimes referred to as segmented designs) to overcome the material's high CTE. Accordingly, single component designs and designs utilizing generally less expensive cordierite materials are sought.
Generally, diesel particulate filters include alternate cell channels plugged on opposite faces to force the engine exhaust gas to pass through the porous walls of the filter. Such filters typically include a catalyst coating, such as an oxidation catalyst, on their surface. Various particulate filters are described in U.S. Pat. Nos. 4,329,162; 4,390,355; 4,416,676; 4,509,966; and 4,840,827, for example.
As the layer of soot collects on the surfaces of the inlet channels of the particulate filter, the lower permeability of the soot layer causes a gradual rise in the back pressure of the filter against the engine (soot loaded back pressure), causing the engine to work harder. Once the soot in the filter has accumulated to some level, the filter must be regenerated by burning out the soot, thereby restoring the back pressure again to lower levels. However, the coated filter (and coated back pressure drop) sets the floor for how “low” the back pressure can be made to be. Thus, it should be recognized that wash coating of the filter raises the back pressure as compared to an uncoated filter. Moreover, it should also be recognized that the contribution of the uncoated filter to the overall back pressure is not insignificant.
Thus, it should be recognized that a significant problem associated with diesel particulate filters is backpressure. Moreover, the coated pressure drop may exceed the uncoated pressure drop by a significant margin. Thus, it would be considered a significant advancement to obtain a particulate filter which has lower wash-coated pressure drop, such that the back pressure against the engine remains low. Furthermore, any across-the-board reduction in the uncoated pressure drop would also be desirable. These backpressure reductions should be accomplished while retaining good filtration efficiency, strength and thermal shock resistance in the diesel particulate filter.